The present invention relates to rigid PVC resin powder compositions adapted for the fabrication of pipe and other extruded articles, conduit, and more particularly relates to rigid PVC resin powder compositions of improved free-flow characteristics and processing performance which are prepared from bulk- or mass-polymerized PVC resin.
As known in the art, the manufacture of PVC resin in mass, as opposed to that prepared via aqueous suspension or emulsion techniques, is conducted in the absence of solvents, diluents and additives other than catalysts. "In mass" is synonomous in meaning to the term "in bulk." Polymerization processes in mass or bulk which are commercially practiced on a large scale at present are autoclave processes, it being advantageous in most instances to carry out the process with the vinyl chloride monomer in the liquid phase and vinyl chloride is a gas at ordinary temperatures. These liquid-phase processes normally are conducted in two stages beginning with a homogeneous liquid medium of very low viscosity, which medium is gradually converted to an essentially solid phase with increasing monomer conversion. Liquid phase bulk polymerization processes are described, for example, in U.S. Pat. Nos. 3,522,227; 3,535,298; 3,539,544; and 3,759,885, among others.
Mass-polymerized PVC resin is generally reputed to be of greater purity than that made by either a suspension or emulsion process since no suspending agent or emulsifier is incorporated in the polymerization mixture. In compounding, it is oftentimes advantageous to use mass-polymerized PVC rather than suspension-polymerized material, since the higher bulk density of the mass PVC allows for larger resin charge sizes per mixing cycle thus providing reduced compounding time overall. Also, the higher bulk density of the resin and its more uniform particle structure provide faster extrusion rates for its compounds. However, when mass-polymerized PVC is employed in the preparation of highly filled powder compositions, particularly rigid formulations, caking of material on the walls of the mixer and/or of the cooling chamber has consistently occurred. This buildup has, of course, lengthened mixing cycles, slowing down compounding significantly. Finally, when filled powder compositions of bulk-polymerized PVC have been extruded to finished plastic articles, the compositions are not free flowing and buildup of the composition on the auger feed screw has developed, making it necessary to continually increase the screw speed. Eventually, total loss of feed has occurred and operations have been halted to clean the equipment.